Percy is a little green saddle tank engine who was brought to Sodor to help run the railway during Gordon, Henry and James' strike. He is Thomas' best friend and one of the youngest engines on the North Western Railway and is quite cheeky, often getting himself into trouble with his attempts to play tricks on the other engines, or just being his clumsy self! Percy's favorite job is to deliver the mail and he works on Thomas' Branch Line as a goods engine.

Percy is a rather cheeky, but kind engine. He normally behaves well, yet he also gets himself in trouble by jumping into situations without adequately thinking them through first. He loves teasing other engines, especially bigger ones such as Gordon and James, but is always brought down to earth in due course by anyone who gives him the moral to learn, especially Sir Topham Hatt. He can also be somewhat timid.